1. Field of the Invention
The present invention relates to a UV sensor comprising an optional housing having an entry opening for UV radiation, a photodetector disposed in the housing for measurement of UV radiation optionally incident through the housing entry opening, and an SiO.sub.2 -containing dispersive element disposed ahead of the photodetector in the direction of incident UV radiation.
2. Description of the Prior Art
UV-irradiators or UV-lasers are used, e.g., in the curing of paints, in the photolytically assisted removal of coating material, in the disinfection of surfaces, liquids, and gases, and in tests of aging and weathering. UV sensors are commonly used to measure doses of UV radiation and to monitor the functioning of UV radiation sources.
In the measurement of UV radiation the sensor material or the sensor itself may suffer radiation damage from the radiation, which is of a relatively high energy. Further, and in many applications, there is the problem of adequately and accurately measuring radiation incident on a UV sensor from all directions.
A general UV sensor is disclosed in Ger. 3,902,028, incorporated herein by reference. This reference sensor consists of a cylindrical housing in which a vacuum photocell with a cesium chloride photocathode is mounted. The housing has an entry opening for incident UV radiation, which opening is partially blocked by a perforated plate which serves to reduce the intensity of the incident radiation. A dispersive element in the form of a diffusing screen is disposed between the perforated plate and the photocell. The diffusing screen comprises polycrystalline quartz material, preferably fritted quartz.
The diffusing screen of the reference sensor provides diffuse scattering of the light, thereby further reducing the intensity of the light incident on the photocell and appreciably reducing the dependence of the measurement on the direction of the incident light. The diffusive action of the diffusing screen is attributable to dispersion of the UV light at the grain boundaries of the quartz crystals and the particle boundaries of the fritted quartz. Accordingly, the diffusive properties of the diffusing screen depend on the grain size distribution of the quartz material used, and, fundamentally, on the manufacturing conditions of the screen. Thus, e.g., a very high sintering temperature results in disappearance of the grain boundaries and an increase in the transmissivity of the diffusing screen. At a lower sintering temperature the light attenuation of the diffusing screen is higher and the transmissivity is lower; further there is the hazard that open pores will survive between particles of the fritted glass which will compromise the gas-tightness of the UV sensor.
Because quartz crystal is not isomorphic as regards light transmission, light refraction and light diffraction, the diffusive properties of known diffusing screens depend on the fortuitous orientation of the quartz grains. Accordingly, it is difficult to provide reproducible optical properties, strength, solidity, and density of the diffusing screen. Since known diffusing screens have low strength they must be handled with care after sintering and are difficult or impossible to machine or otherwise process mechanically. In addition, soil particles accumulate in the open pores remaining on the surface of the diffusing screen, which particles are difficult to remove. Thus, with time, the transmissivity of prior art diffusing screens deteriorates in an indefinite, unpredictable manner. Contributing to this decrease in transmissivity is material damage caused by the energetic UV radiation in the crystal frit.